Homogeneous and reusable superacid polymer catalyst useful for the synthesis of 5-hydroxymethylfurfural from glucose

ABSTRACT

A superacid polymeric catalyst having both Lewis acidity and Brønsted acidity is described, along with methods of making and methods of using the same.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a national stage application filed under 35 U.S.C. §371 of international application PCT/US2018/057984, filed under theauthority of the Patent Cooperation Treaty on Oct. 29, 2018, whichclaims priority to U.S. Provisional Application No. 62/578,659 filedunder 35 U.S.C. § 111(b) on Oct. 30, 2017, the disclosure of which isincorporated herein by reference in its entirety for all purposes.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This invention was made with no government support. The government hasno rights in this invention.

BACKGROUND OF THE INVENTION

The synthesis of 5-hydroxymethylfurfural (HMF) from glucose typicallyrequires both Brønsted acid sites and Lewis acid sites. Lewis acid sitesare responsible for the isomerization of glucose to fructose, whileBrønsted acid sites are responsible for the dehydration of fructose toHMF. Thus, combinations of Brønsted and Lewis acid catalysts (forexample, HCl and AlCl₃) are generally used to produce HMF from glucose.However, reutilization of such catalysts is complicated due to theirsolubility in the reaction medium. Heterogeneous catalysts containingboth Brønsted acid sites and Lewis acid sites have also been prepared(for example, Sn—W oxide), but they are not usually as active as theirhomogeneous counterparts because not all active sites are exposed. Thus,there is a need in the art for new and improved catalysts for theproduction of HMF and other processes.

SUMMARY OF THE INVENTION

In a first broad aspect, described herein is a composition comprisingFormula A:

where m is an integer. In certain embodiments, m ranges from about 5,000to about 85,000.

In certain embodiments, the polystyrene chain has a molecular weightfrom about 10,000 to about 1,500,000.

The composition is soluble in polar solvents. In certain embodiments,the composition is dissolved in a polar solvent selected from the groupconsisting of water, gamma-valerolactone (GVL), dimethyl sulfoxide(DMSO), and DMSO-water systems.

In certain embodiments, the composition is dissolved in a polar solventcomprising water, and a MIBK+2-butanol mixture is used as an organicphase for HMF extraction.

In certain embodiments, composition includes Brønsted acid sites andLewis acid sites at a estimated Brønsted:Lewis ratio of up to about90:10.

In certain embodiments, the estimated Brønsted:Lewis ratio is about80:20.

In certain embodiments, the estimated Brønsted:Lewis ratio is about70:30.

In certain embodiments, the estimated Brønsted:Lewis ratio is about60:40.

In certain embodiments, the estimated Brønsted:Lewis ratio is about50:50.

In certain embodiments, the estimated Brønsted:Lewis ratio is about40:60.

In certain embodiments, the estimated Brønsted:Lewis ratio is about30:70.

In certain embodiments, the estimated Brønsted:Lewis ratio is about20:80.

In certain embodiments, the estimated Brønsted:Lewis ratio is about10:90.

In certain embodiments, the composition further comprises nanoparticles,nanofibers, or nanosheets. In certain embodiments, the nanoparticlescomprise alumina and/or carbon. In certain embodiments, the nanofiberscomprise carbon. In certain embodiments, the nanosheets comprisegraphene.

In certain embodiments, the composition further comprises a monomerwhich increases the hydrophilicity of the composition.

In another broad aspect, described herein is a composition comprising apoly(styrenesulfonic acid)-based (PSSA) polymer having both Lewis acidsites and Brønsted acid sites, wherein the composition is soluble inpolar solvents.

In certain embodiments, the composition is made by ion exchange betweenPSSA and AlCl₃ in a liquid medium.

In certain embodiments, the composition is made by ion exchange betweenPSSA and one or more of SnCl₄, TiCl₄, BF₃, MoS₂, ZnCl₂, VCl₄, NiCl₂,GaCl₃, GeCl₄, AsCl₂, BCl₃, SiCl₄, SbCl₃, PCl₃, or Et₂AlCl₃.

In another broad aspect, described herein is a method of producing acatalyst, the method comprising adding a Lewis acid to soluble PSSA in aliquid medium to produce a superacid catalyst.

In certain embodiments, the Lewis acid is AlCl₃.

In certain other embodiments, the Lewis acid is one of SnCl₄, TiCl₄,BF₃, MoS₂, ZnCl₂, VCl₄, NiCl₂, GaCl₃, GeCl₄, AsCl₂, BCl₃, SiCl₄, SbCl₃,PCl₃, or Et₂AlCl₃.

In certain embodiments, the liquid medium comprises a mixture ofmethanol and ethanol.

In certain embodiments, the mixture comprises a ratio ofmethanol:(methanol+ethanol) ranging from about 0.5 to about 0.75 byvolume.

In certain embodiments, the ratio is about 0.6methanol:(methanol+ethanol) by volume.

In another broad aspect, described herein is a method of preparing5-hydroxymethylfurfural (HMF), where the method comprises isomerizingglucose to fructose and dehydrating fructose in a feedstock with asingle catalyst to produce HMF, wherein the catalyst comprises apoly(styrenesulfonic acid)-based (PSSA) polymer having both Lewis acidsites and Brønsted acid sites.

In certain embodiments, the catalyst comprises Formula A:

where m is an integer.

In certain embodiments, the polystyrene chain has a molecular weightfrom about 10,000 to about 1,500,000.

In certain embodiments, m ranges from about 5,000 to about 85,000.

In certain embodiments, the isomerization and dehydration are conductedin a solvent comprising water, gamma-valerolactone (GVL), dimethylsulfoxide (DMSO), a water-DMSO system, or in a biphasic aqueous-organicsystem comprising water-(MIBK+2-butanol).

In certain embodiments, the method further comprises converting the HMFinto one of dimethylfuran (DMF), adipic acid, 1,6-hexanediol, levulinicacid, caprolactam, 2,5-dimethylfuran, 5-hydroxymethylfuronic acid,3,5-dihydroxymethylfuran, 5-hydroxy-4-keto-2-pentenoic acid, or2,5-furandicarboxylic acid (FDCA).

In certain embodiments, the feedstock comprises a mixture of the glucosewith fructose.

In another broad aspect, described herein is a method of making acatalyst, where the method comprises:

i) dissolving PSSA in anhydrous methanol to form a PSSA-methanolsolution;

ii) dissolving AlCl₃ in anhydrous ethanol to form an AlCl₃-ethanolsolution;

iii) adding the AlCl₃-ethanol solution to the PSSA-methanol solution tocreate a mixture;

iv) stirring the mixture for a period; and

v) filtering the mixture with a membrane to separate the catalyst.

In certain embodiments, the membrane comprises polyethersulfone.

In certain embodiments, the method further comprises stirring one orboth of the PSSA-methanol solution and the AlCl₃-ethanol solution priorto the adding step iii).

In certain embodiments, the method further comprises adjusting theamount of AlCl₃ dissolved in the anhydrous ethanol to adjust the amountof Lewis acid sites in the catalyst.

In certain embodiments, the mixture comprises a ratio ofmethanol:(methanol+ethanol) ranging from about 0.5 to about 0.75 byvolume.

In another broad aspect, described herein is a solution comprisingsoluble AlCl₃ and soluble polystyrenesulfonic acid (PSSA) in a mixtureof ethanol and methanol, where the mixture comprises a ratio ofmethanol:(methanol+ethanol) ranging from about 0.5 to about 0.75 byvolume, the AlCl₃ is present at a concentration of at least 0.001 g/mLmixture, and the PSSA is present at a concentration of at least 0.012g/mL mixture.

In certain embodiments, the solution has the ratio ofmethanol:(methanol+ethanol) about 0.6 by volume.

In another broad aspect, described herein is a kit for making a catalysthaving a first container housing a PSSA polymer; and a second containerhousing AlCl₃. In certain embodiments, the kit further includes at leastone solvent.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 : Production of PSSA-AlCl₃ (Formula A) through ion exchangebetween PSSA and AlCl₃.

FIGS. 2A-2B: Synthesis of polystyrene sulfonic acid (PSSA) by ionexchange of sodium polystyrene sulfonate (PSSS) with a sulfonic resin(FIG. 2A), and photograph of the PSSA so obtained (FIG. 2B).

FIG. 3 : Conversion of glucose to HMF by a combinedisomerization/dehydration pathway.

FIG. 4 : Non-limiting example compounds made from HMF.

FIG. 5 : Phase diagram to optimize the methanol:ethanol volume ratio foran effective ion exchange. Stock: methanol+ethanol total volume.

FIG. 6 : Weight loss % vs temperature of PSSA and series of PSSA-AlCl₃catalysts.

FIG. 7 : Derivative weight loss % vs temperature of PSSA and series ofPSSA-AlCl₃ catalysts.

FIG. 8 : ATR spectra of PSSA and series of PSSA-AlCl₃ catalysts.

FIG. 9A-9C: Conversion of fructose to HMF with PSSA, H₂SO₄, andAmberlyst 15®. FIG. 9A shows fructose conversion, and FIG. 9B shows HMFyield. Conditions: (MIBK+2-butanol): water=(6.3 mL+2.7 mL): 1.5 mL, 150mg fructose, 150° C., 1000 rpm, (32.4 mg PSSA, 34.0 mg Amberlyst®, 1.5mL of a 0.06 M H₂SO₄ solution, all corresponding to 0.175 mmol H⁺).Lines have been added to guide the eye. FIG. 9C shows turnoverfrequencies for production of HMF with PSSA catalyst: Reutilizationexperiment. Conditions: (MIBK+2-butanol): water=(4.2 mL+1.8 mL): 3 mL,300 mg fructose, 150° C., 940 rpm, 100 mg PSSA, 30 min. The red line inFIG. 9C represents the mean value±standard deviation for the six runs.

FIG. 10A-10C: Conversion of glucose to HMF with PSSA and PSSA-AlCl₃catalysts. FIG. 10A shows glucose conversion, and FIG. 10B shows HMFyield. Conditions: (MIBK+2-butanol): water=(6.3 mL+2.7 mL): 1.5 mL, 150mg glucose, 150° C., 1000 rpm, (32.4 mg PSSA, 35 mg PSSA-AlCl₃ 90:10,37.8 mg PSSA-AlCl₃ 80:20, 38 mg PSSA-AlCl₃ 70:30, 39.5 mg PSSA-AlCl₃60:40, 40 mg PSSA-AlCl₃ 50:50, 42.7 mg PSSA-AlCl₃ 40:60, 44.5 mgPSSA-AlCl₃ 30:70, 64.6 mg PSSA-AlCl₃ 20:80, all corresponding to 0.175mmol H⁺). Lines have been added to guide the eye. FIG. 10C showsturnover frequencies for production of HMF with PSSA 70:30catalyst:Reutilization experiment. Conditions: (MIBK+2-butanol):water=(4.2 mL+1.8 mL): 3 mL, 300 mg glucose, 150° C., 940 rpm, 140 mgPSSA-AlCl₃ 70:30, 2 h. The red line in FIG. 10C represents the meanvalue±standard deviation for the five runs.

DETAILED DESCRIPTION

The disclosures of these publications, patents, and published patentspecifications are hereby incorporated by reference into the presentdisclosure in their entirety to more fully describe the state of the artto which this invention pertains.

Provided is a reusable polymer catalyst containing both Brønsted andLewis acid sites. The catalyst has been demonstrated to be effective inthe production of 5-hydroxymethylfurfural from glucose in a one-potsynthesis. The catalyst is further useable in a wide variety of otherapplications (e.g., synthesis of furfural from xylose).

Poly(styrenesulfonic acid) (PSSA) combines the advantages of bothhomogeneous and heterogeneous catalysis. PSSA is soluble in polarsolvents. Therefore, all acidic sulfonic groups are readily accessible.In addition, PSSA cannot be deactivated through coking because there isno surface for the carbonaceous species to be deposited. Heterogeneouscatalysts are generally subjected to coking. At the same time, PSSA, dueto its high molecular weight, can be easily recovered by ultrafiltrationfor further utilization. PSSA can be prepared by, for instance, by ionexchange of sodium polystyrenesulfonate (PSSS) with a sulfonic resin, asdepicted in FIG. 2A. PSSA can be obtained by sulfonation of polystyrenewaste (e.g., yogurt packaging or expanded polystyrene), which is anadditional advantage from an environmental point of view. This polymercatalyst demonstrated high activity in several biomass conversionreactions that require Brønsted acid sites, such as: synthesis ofbiodiesel from vegetable oil, dehydration of xylose to furfural, andfurfural oxidation to maleic and succinic acids. However, in the contextof HMF production from glucose, PSSA can catalyze the dehydration step,but generally only to a limited extent the isomerization step, and thusis not used as the only catalyst to produce HMF from glucose.

In accordance with the present disclosure, Lewis acid functionality isadded to PSSA to create a superacid catalyst, PSSA-AlCl₃. In general,the PSSA-AlCl₃ catalyst composition has the structural formula ofFormula A:

m ranges from about 5,000 to about 85,000. In certain embodiments, thepolystyrene chain has a molecular weight from about 10,000 to about1,500,000. However, it is understood that other values m are entirelypossible and encompassed within the scope of the present disclosure. Thesuperacid catalyst is useful, for instance, for catalyzing a one-potsynthesis of hydroxymethylfurfural (HMF) from glucose in a reusablemanner.

Although PSSA-AlCl₃ is described for exemplary purposes, Lewis acidfunctionality can be added to PSSA through the addition of Lewis acidsites other than AlCl₃. For example, Lewis acid sites can beincorporated into PSSA using Lewis acids such as SnCl₄, TiCl₄, BF₃,MoS₂, ZnCl₂, VCl₄, NiCl₂, GaCl₃, GeCl₄, AsCl₂, BCl₃, SiCl₄, SbCl₃, PCl₃,Et₂AlCl₃, or the like, in the same manner as described herein for AlCl₃(i.e., ion exchange) to similarly produce superacid catalysts. ThoughAlCl₃ is described herein for illustrative purposes, such othercatalysts are encompassed within the present disclosure.

Since PSSA already contains Brønsted acid groups, Lewis acid sites areadded to PSSA to synthesize a superacid polymer catalyst (e.g.,PSSA-AlCl₃) useful for conversion of glucose to HMF via one-potsynthesis. The PSSA-AlCl₃ catalyst is soluble in polar solvents (i.e.,is homogeneous) with high molecular weight for an easy recovery byultrafiltration for further reutilization. Moreover, the number ofactive sites on the catalyst can be customized. It is to be noted that,as used herein “estimated” means “theoretical based on the amountsadded.” For example, in certain embodiments, Formula A includes aestimated Brønsted:Lewis ratio of about 10:90, about 20:80, about 30:70,about 40:60, about 50:50, about 60:40, about 70:30, about 80:20, orabout 90:10. The ratio of Brønsted:Lewis sites can be customized for thedesired application.

Formula A can be prepared through the addition of AlCl₃ to solublepoly(styrenesulfonic acid) (PSSA) by ion exchange in liquid medium.(FIG. 1 .) PSSA may be obtained by sulfonation of polystyrene waste orby ion exchange of polystyrene sulfonate, such aspoly(sodium-4-styrenesulfonate) (PSSS) with a sulfonic resin or H₂SO₄.PSSA is a soluble polymer containing only Brønsted acid sites that hasdemonstrated high activity in several biomass conversion reactions,namely, synthesis of biodiesel from vegetable oil, dehydration of xyloseto furfural, and furfural oxidation to maleic and succinic acids.Sulfonic resins have previously had Lewis acid sites incorporated.However, such previous catalysts were not soluble in polar solvents.

An acid resin, such as, but not limited to, Amberlyst 15®, may be usedas a sulfonic resin for ion exchange to produce PSSA from PSSS. Forexample, Amberlyst 15® may be added to a solution of PSSS. The sulfonicresin can then be removed from the solution by filtration, leavingbehind a PSSA solution that can be heated to evaporate water in order torecover solid PSSA. The solid PSSA can be dissolved in a solvent such asmethanol, and optionally subjected to ultrafiltration (such as with apolyethersulfone membrane) to remove polymer chains having a smallerthan desired size. Other methods of producing PSSA for ion exchange withAlCl₃ are possible and entirely encompassed within the presentdisclosure.

To conduct the ion exchange between AlCl₃ and PSSA, AlCl₃ is dissolvedin a suitable solvent, such as ethanol, to form a solution, and thisAlCl₃ solution is added to a solution of PSSA in a suitable solvent,such as methanol. Optionally, this is conducted dropwise with constantstirring. Stirring may continue for a period of time, such as severalhours, at room temperature after the solutions have been fully combined,in order to complete the ion exchange. Once completed, the mixture canbe subjected to ultrafiltration, such as with a polyethersulfonemembrane, and dried to recover PSSA-AlCl₃. In order to adjust the ratioof Brønsted:Lewis sites in the product, different amounts of AlCl₃ areused in the ion exchange.

Notably, AlCl₃ is not soluble in methanol, but the PSSA polymer is.Furthermore, PSSA is not very soluble in ethanol, though AlCl₃ is.Accordingly, a mixture of ethanol and methanol may be used to solubilizeboth AlCl₃ and PSSA for the ion exchange to prepare Formula A. In someembodiments, this mixture is at a methanol:(methanol+ethanol) ratioranging from about 0.5 to about 0.75 by volume. In one non-limitingexample, the mixture comprises a 0.6 ratio by volume ofmethanol:(methanol+ethanol). However, other methods of adding AlCl₃ tothe soluble PSSA are possible and entirely encompassed within the scopeof the present disclosure. Furthermore, in order to maintain thesolubility of the polymer for an increased performance, the degree ofsulfonation is typically kept above 30% when adding the Lewis acidfunctionality. However, it is understood that this is not strictlynecessary to produce Formula A.

Homogeneous catalysts are generally more active than their heterogeneouscounterparts because their solubility in the reaction medium contributesto access of reactants to all active sites. However, heterogeneouscatalysts are commonly preferred because heterogeneous catalysts can beeasily recovered from the reaction medium and be reused. The catalyst ofFormula A combines the advantages of both homogeneous and heterogeneouscatalysts. The catalyst of Formula A is soluble in polar solvents;therefore, it acts as a homogeneous catalyst, and all the acid sites areeasily reachable and exposed for catalysis. At the same time, beingsoluble means the catalyst of Formula A cannot be deactivated throughcoking because there is no physical surface for the carbonaceous speciesto be deposited. This is especially important for biomass conversionreactions, since this mode of deactivation is a very common problem.Furthermore, due to its high molecular weight, the catalyst of Formula Acan be easily recovered by ultrafiltration to be reused.

Non-limiting examples of polar solvents that Formula A is soluble ininclude water, methanol, gamma-valerolactone (GVL), dimethyl sulfoxide(DMSO), and water-DMSO systems. Advantageously, GVL is also considered agreen solvent. In one non-limiting example, the reaction is carried outin an aqueous-organic biphasic system. In one non-limiting example, thecatalyst of Formula A is dissolved in water with a combination of MIBKand 2-butanol as the organic phase for HMF extraction. In anothernon-limiting example, the catalyst of Formula A is dissolved in asolvent composed of a DMSO-water system.

There are many types of reactions that benefit from a homogeneous andreusable catalyst having both Brønsted and Lewis acid functionalities.One non-limiting example is the production of 5-hydroxymethylfurfural(HMF) from glucose, which requires Lewis acid sites for isomerization ofglucose to fructose, and Brønsted acid sites for dehydration of fructoseto HMF. (FIG. 3 .) As described below, the effectiveness of the catalystof Formula A has been demonstrated in this tandem reaction. However, thecatalyst of Formula A may be employed as a catalyst in any reactionwhich involves an acid catalyst, including, but not limited to,esterifications, isomerizations, alkylations, polymerizations, crackingreactions, acylations, etherifications, acetalizations, nitrations, anddisproportionations.

As noted above, the catalyst of Formula A can be recovered from areaction medium through methods such as ultrafiltration. Ultrafiltrationis a type of membrane filtration in which pressure forces a liquidagainst a semipermeable membrane, which is a thin layer of materialcapable of separating substances when a driving force is applied acrossit. Ultrafiltration is applied in a variety of applications, but mainlyin the filtration of biomolecules of interest in medical and biochemicalapplications. Its viability has also been shown in polymer applications.Once the polymer is retained in the membrane, it can be liberated byre-dissolution in the reaction medium and be reutilized provided thatthe catalyst is not deactivated during use. The membranes here used forultrafiltration can recover catalysts with a molecular weight as low asabout 5 kDa. As shown in the examples herein, the catalyst of Formula Acan be recovered from reaction media by ultrafiltration and reusedwithout deactivation.

Formula A may be further modified in a variety of ways encompassedwithin the present disclosure. For example, PSSA-AlCl₃ may be obtainedby addition of AlCl₃ leading to crosslinking (Formula A) or bycopolymerization of monomers containing Brønsted and Lewis acid sites.Further, PSSA-AlCl₃ may be anchored to nanoparticles, nanofibers, ornanosheets to allow for conventional filtration for easier recovery. Thesolubility of the polymer chains on the reaction medium eliminates thedeactivation by coking. Additionally, PSSA-AlCl₃ can be modified withmonomers that improve the hydrophilicity of the material. This helpsincrease the amount of Lewis acid sites on the polymer while maintainingit soluble, which improves, for example, the synthesis of HMF fromglucose.

HMF can be produced from glucose by a tandem reaction which involves theisomerization of glucose to fructose followed by the dehydration offructose to HMF. (FIG. 3 .) A one-pot synthesis of HMF involves theisomerization of glucose to fructose in the presence of a Lewis acidcatalyst followed by the dehydration of fructose to HMF using a Brønstedacid catalyst. The most common Lewis acid catalysts conventionally usedin this reaction are AlCl₃, SnCl₄, CrCl₃, GaCl₃, InCl₃, and YbCl₃, amongothers. On the other hand, mineral acids such as HCl and H₂SO₄,zeolites, and sulfonic resins have been used as a source of Brønstedacid sites. For the synthesis of HMF from glucose, combinations of bothLewis and Brønsted acid sites are typically used, as well as catalystscontaining both functionalities.

H₂SO₄ is a very active catalyst for the dehydration of fructose to HMF,but it cannot be reused due to its homogeneous nature. Similarly, AlCl₃cannot be easily recovered and reused because it is dissolved in thereaction medium. For this reason, heterogeneous catalysts are usuallypreferred. Among the heterogeneous acid catalysts containing —SO₃Hgroups, Amberlyst 15®, Amberlyst 70®, Amberlyst 38®, and Dowex® havebeen used to convert fructose into HMF due to their large number ofBrønsted acid sites (—SO₃H). However, the major drawback of sulfonicresins is that they deactivate through leaching and/or coking when usedin reactions at high temperature and pressure. In addition, to match thenumber of acid sites with that on sulfuric acid, large amounts ofsulfonic resins are required in the reaction.

Amberlyst 15® and Amberlyst 70® have also shown promising results in thepresence of other solvents, such as dimethylformamide (DMF),tetrahydrofuran (THF), water, dioxane, and ionic liquids. However, adisadvantage of carrying out this reaction only in an aqueous phase isthat products of hydration of HMF, such as formic and levulinic acids,are easily formed. The addition of poly(1-vinyl-2-pyrrolidinone) (PVP)or DMSO to the system reduces the amount of side products formed. Inaddition to sulfonic resins, other heterogeneous catalysts such aszeolites, mesoporous catalysts, and polymer catalysts containingsulfonic groups have been used.

Although fructose is commonly used to produce HMF, glucose is preferredover fructose due to its higher abundance and lower cost compared tofructose. Thus, the production of HMF in one-pot from glucose instead offrom fructose is more cost-effective. This efficiency is amplified bythe fact that the PSSA-AlCl₃ catalyst is recoverable and reusable. ThePSSA-AlCl₃ catalyst of Formula A is capable of catalyzing this one-potsynthesis of HMF from glucose. Furthermore, because Formula A cancatalyze both the isomerization of glucose to fructose and thedehydration of fructose to HMF, Formula A can be used to produce HMFfrom a feedstock that contains a mixture of glucose and fructose. Theone-pot synthesis can be conducted in, for example, solvents such aswater, GVL, DMSO, or a mixture of DMSO and water. However, anaqueous-organic biphasic system, such as water-(MIBK+2-butanol) isadvantageous for extracting HMF as soon it is formed to minimize theoccurrence of side reactions. In such solvents, the PSSA-AlCl₃ catalystof Formula A is soluble and the reaction can proceed more efficiently.

HMF is a platform molecule, useful for producing a variety of valuablechemicals. (FIG. 4 .) HMF, together with furfural and2,5-furandicarboxylic acid (FDCA), are derivatives of furan compoundswhich are among the top value-added bio-based chemicals currentlyproduced. HMF derivatives, such as FDCA as a substitute of terephthalicacid in the PET industry, or adipic acid for the nylon industry, are indemand. HMF is also useful as an intermediate for the production of thebiofuel dimethylfuran (DMF) and other molecules such as levulinic acid,2,5-diformylfuran, 3,5-dihydroxymethylfuran, and5-hydroxy-4-keto-2-pentenoic acid. Thus, the method of preparing HMFdescribed herein can be but one step of a multi-step synthesis for aplethora of downstream products.

The compositions and methods described herein may be embodied as partsof a kit or kits. A non-limiting example of such a kit is a kit formaking a catalyst of Formula A, the kit comprising a PSSA polymer andAlCl₃ in separate containers, where the containers may or may not bepresent in a combined configuration. Many other kits are possible, suchas kits further comprising at least one solvent for solubilizing PSSAand/or AlCl₃, and/or at least one polar solvent for dissolving aPSSA-AlCl₃ catalyst. The kits may further include instructions for usingthe components of the kit to practice the subject methods. Theinstructions for practicing the subject methods are generally recordedon a suitable recording medium. For example, the instructions may bepresent in the kits as a package insert or in the labeling of thecontainer of the kit or components thereof. In other embodiments, theinstructions are present as an electronic storage data file present on asuitable computer readable storage medium, such as a flash drive. Inother embodiments, the actual instructions are not present in the kit,but means for obtaining the instructions from a remote source, such asvia the internet, are provided. An example of this embodiment is a kitthat includes a web address where the instructions can be viewed and/orfrom which the instructions can be downloaded. As with the instructions,this means for obtaining the instructions is recorded on a suitablesubstrate.

EXAMPLES Example I—Synthesis and Characterization of PSSA-AlCl₃

Sulfonic groups in PSSA were partially replaced by AlCl₃ whilemaintaining a degree of sulfonation higher than 30% to keep the polymersoluble in reaction medium for HMF production from glucose. The resultwas a superacid PSSA-AlCl₃ catalyst produced in liquid medium underinert atmosphere at room temperature.

Preparation of PSSA-AlCl₃ Superacid Catalyst by Ion Exchange

PSSA was prepared from poly(sodium-4-styrenesulfonate) (PSSS) (suppliedby Sigma-Aldrich (25 wt %, approx. MW of the polymer 200 kDa)) as theprecursor. The PSSS was transformed to PSSA by ion exchange using anacid resin, Amberlyst 15® (H⁺ capacity=4.7 meq·g⁻¹). 120 g of Amberlyst15® was crushed and added into 600 mL of an aqueous solution of PSSS.The mixture was stirred overnight at room temperature to maximize theexchange (FIG. 2A). The sulfonic resin was then removed from thesolution by conventional filtration. The filtered PSSA solution obtainedwas then heated in an oven for 8 h at 70° C. to evaporate water. Thedried solid was then dissolved in about 900 mL of methanol undervigorous stirring until dissolution. The PSSA-methanol solution wassubjected to ultrafiltration using a 5 kDa polyethersulfone membrane(Millipore Biomax) to remove polymer chains of smaller size. The firststep of the ultrafiltration technique involved removing the glossyglycerine layer present on the membrane by passing about 700 mL of waterthrough it. The second step involved the ultrafiltration of themethanol-PSSA solution through the same 5 kDa membrane in a cellpressurized to 25 psi with N₂ under stirring. The ultrafiltered PSSAsolution was dried overnight at 50° C. A photograph of the PSSA obtainedafter drying is shown in FIG. 2B.

To carry out the ion exchange with AlCl₃ in liquid phase under inertatmosphere, PSSA was dissolved in anhydrous methanol and AlCl₃ wasdissolved in anhydrous ethanol. Notably, the solubility of PSSA inethanol and the solubility of AlCl₃ in methanol are each low. To betterunderstand the solubility of both PSSA and AlCl₃ in mixtures of methanoland ethanol, some preliminary experiments were carried out first. Allthe experiments were performed in a VAC glovebox to ensure that AlCl₃was not oxidized or hydrated to form either Al₂O₃ or AlCl₃.6H₂O. Thevolume of ethanol used was fixed and the amount of methanol was varied,as well as the amount of PSSA or AlCl₃ dissolved. Using the results ofthese experiments, a phase diagram was drawn (FIG. 5 ) and the region inwhich both PSSA and AlCl₃ were soluble in the ethanol-methanol mixturewas taken as the optimum to carry out the ion exchange. The use of thismethanol:ethanol ratio avoids the precipitation or formation ofemulsions, making the ion exchange more efficient. Once themethanol:ethanol ratio was optimized, PSSA was dissolved in theappropriate amount of anhydrous methanol, and AlCl₃ was dissolved inanhydrous ethanol, in separate vials. To maintain a uniform solubility,the PSSA-methanol and AlCl₃-ethanol solutions were independently stirredfor 1 h. Then, the AlCl₃-ethanol solution was added to the PSSA-methanolsolution dropwise using a peristaltic pump at a rate of 0.6 mL/min understirring at 1000 rpm. The mixture was then stirred for 3 h at roomtemperature for the ion exchange to take place. Once completed, themixture was ultrafiltered using a 5 kDa Biomax polyethersulfonemembrane. The retentate was dried overnight at 50° C. The appearance ofPSSA-AlCl₃ was similar to that of PSSA but its color was less intense.

Thermogravimetric Analysis (TGA) of PSSA-AlCl₃ Catalysts

TGA of PSSA-AlCl₃ samples were carried out at the Center for Materialsand Sensor Characterization (CMSC) of the University of Toledo using aTA TGA instrument with a heating ramp of 10° C./min starting from roomtemperature to 800° C. under flow of nitrogen. ˜7 mg of catalyst wasused to analyze the thermal properties of these materials.

Amount of BrøNsted Acid Sites in PSSA-AlCl₃ Catalysts

The amount of Brønsted acid sites on the PSSA-AlCl₃ catalysts wasanalyzed using acid-base titration with 0.1 N NaOH. The NaOH solutionwas first standardized using potassium phthalate. 15 mg of PSSA-AlCl₃catalyst was dissolved in approximately 10 mL of water and titratedagainst NaOH solution using phenolphthalein as an indicator. Differentcatalysts with varied Brønsted:Lewis acid site ratios were obtained byusing different amounts of AlCl₃. Samples were labelled as PSSA-AlCl₃B:L, where B stands for the estimated percentage of Brønsted acid sitespresent in the catalyst and L stands for the number of Brønsted acidsites estimatedly substituted by Lewis acid sites by ion exchange.

¹H and ¹³C NMR of PSSA-AlCl₃ Catalysts

¹H and ¹³C NMR of PSSA-AlCl₃ catalysts were performed at theInstrumentation Center of the University of Toledo using a BrukerAVANCE-600 NMR equipment. Samples for ¹H NMR were prepared by dissolving20 mg of PSSA-AlCl₃ in 0.5 mL of deuterated methanol under stirring for1 h. The parameters of operation for ¹H NMR were AQ mode: DQD, TD:65536; DS: 2, AQ: 2.72, NS: 16.

Samples for ¹³C NMR sample were prepared by dissolving 20 mg ofPSSA-AlCl₃ in 0.5 mL of deuterated water under stirring for 1 h. Theparameters of operation for ¹³C NMR were SW, 250 ppm, TD: 65536; DS: 20,AQ: 0.98, NS: 1024.

Attenuated Total Reflection (ATR) Analysis of PSSA-AlCl₃ Catalysts

PSSA-AlCl₃ samples were characterized by ATR at the Center for Materialsand Sensor Characterization at the University of Toledo. ˜0.5 mm thinsheets of PSSA and PSSA-AlCl₃ samples were placed on the equipment andanalyzed using ATR with a germanium tip and 124 scans. PSSA-AlCl₃samples were dried under vacuum overnight prior to analysis.

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) Analysis ofPSSA-AlCl₃ Catalysts

The amount of Al incorporated into the polymer and the success of theion exchange were evaluated using an ICP-MS technique. In a typicalanalysis, ˜55 mg of each sample was digested in HNO₃ using a CEM Marsmicrowave. After digestion, samples were filtered to remove particulatematerial. The filtrate was diluted to 3.5% HNO₃ for analysis in ICP-MS(X series 2, Thermo Scientific, MA USA). For quantitative analysis,standards were prepared by using certified ICP-MS standards fromInorganic Ventures. Correlation coefficients for calibration curves wereabove 0.999.

Results and Discussion

Phase Diagram to Optimize the Ion Exchange in Liquid Medium

Since PSSA is soluble in methanol (but insoluble in ethanol), and AlCl₃is soluble in ethanol but slightly soluble in methanol, the optimumliquid mixture to carry out the ion exchange without precipitation orformation of emulsions that would reduce the efficiency of the processwas determined. The volume of ethanol was fixed and different volumes ofmethanol were added. The solubility of PSSA and AlCl₃ independently onthose mixtures was evaluated, and a phase diagram was drawn (FIG. 5 ).The region in which only one phase is observed for both the dissolutionof PSSA and AlCl₃ (green area) can be clearly seen in the phase diagram.0.6 was chosen as the volume ratio for the ion exchange, while usingdifferent volumes of methanol and ethanol to keep the polymer solubleduring ion exchange. Catalysts with different estimated B:L acid sitesratios (90:10, 80:20, 70:30, 60:40, 50:50, 40:60, 30:70, and 20:80) wereprepared. To do so, the amount of PSSA was fixed to 1 g and the amountof anhydrous AlCl₃ used varied between 36 and 290 mg. The volume ofmethanol used varied between 48 mL and 384 mL, and the volume of ethanolbetween 32 and 256 mL.

Thermogravimetric Analysis of PSSA and PSSA-AlCl₃ Catalysts

FIGS. 6-7 compare the results obtained from TGA analysis of PSSA and theseries of PSSA-AlCl₃ catalysts. The weight loss curves for PSSA-AlCl₃catalysts look similar. The decomposition of polymers containing largeramounts of Lewis acid sites happens at higher temperatures. Withoutwishing to be bound by theory, it is believed this is because of thehigher stability of these structures due to their crosslinked nature.

In addition, it can also be observed that the amount of water inPSSA-AlCl₃ catalysts is larger than that of PSSA. The differences aremore easily observed in FIG. 7 , which represents the derivative weightloss for PSSA and series of PSSA-AlCl₃ catalysts. When compared withPSSA, curves of PSSA-AlCl₃ catalysts are displaced to highertemperatures, which indicates a higher stability of those materials. Inaddition, it can be observed that the evolution between 250-400° C.,corresponding to the decomposition of —SO₃H groups in PSSA, is shiftedto higher temperatures when AlCl₃ is added to the catalyst. A reductionof the area of this peak is also observed due to the fact that thenumber of sulfonic groups is being reduced.

The last evolution, corresponding to the decomposition of the PSbackbone, is also displaced to higher temperatures. The larger area ofthis peak on PSSA-AlCl₃ catalysts indicates the decomposition of the PSbackbone together with the PS chains interconnected by a Lewis acidsite. (See FIG. 7 .)

Amount of BrøNsted Acid Sites in PSSA and PSSA-AlCl₃ Catalysts

The amounts of Brønsted acid sites in different PSSA-AlCl₃ catalysts arelisted in Table 1. The amount of Brønsted acid sites is reduced by theaddition of Lewis acid sites by ion exchange.

TABLE 1 Brønsted acid sites in PSSA and series of PSSA-AlCl₃ catalystsSample (labelled Estimated % Estimated % Actual amount of as PSSA-AlCl₃of Brønsted of Lewis Brønsted acid sites Brønsted:Lewis) acid sites acidsites (mmol H⁺ · g cat⁻¹) PSSA 100 0 5.40 PSSA-AlCl₃ 90:10 94.7 5.3 5.00PSSA-AlCl₃ 80:20 88.9 11.1 4.63 PSSA-AlCl₃ 70:30 82.4 17.6 4.60PSSA-AlCl₃ 60:40 75.0 25.0 4.44 PSSA-AlCl₃ 50:50 66.7 33.3 4.38PSSA-AlCl₃ 40:60 57.1 42.9 4.11 PSSA-AlCl₃ 30:70 46.2 53.8 3.94PSSA-AlCl₃ 20:80 33.3 66.7 2.71

¹H and ¹³C NMR of PSSA and PSSA-AlCl₃ Catalysts

PSSA and PSSA-AlCl₃ catalysts were also characterized by NMR (resultsnot shown). No major differences were observed using this technique,only a slight reduction of the degree of sulfonation when AlCl₃ isadded.

ATR of PSSA and PSSA-AlCl₃ Catalysts

FIG. 8 compares the ATR spectra obtained for PSSA and series ofPSSA-AlCl₃ catalysts. All these spectra are very similar. The onlydifferences are in the bands located between 1050 and 1180 cm⁻¹, whichwere assigned to S—O vibrations (˜1080 cm⁻¹), benzene ring planestretching (˜1125 cm⁻¹), and ring C—H in-place bending vibrations (˜1155cm⁻¹), which decrease and increase their intensities respectively whenthe amount of Lewis acid sites is increased.

Inductively Coupled Plasma-Mass Spectrometry (ICP-MS) Analysis of PSSAand PSSA-AlCl₃ Catalysts

Table 2 shows the amount of Al content in the catalysts prepared by ionexchange of PSSA with AlCl₃. The amount of Al in the catalyst increasedwith the amount of AlCl₃ used for the ion exchange.

TABLE 2 ICP-MS results of PSSA and series of PSSA-AlCl₃ catalysts ppb ofAl ppb of Al (μg Al/g catalyst) (μg Al/g catalyst) experimental Catalystestimated (ICP-MS) PSSA-AlCl₃ 90:10 7,285 7,753 PSSA-AlCl₃ 80:20 14,57016,994 PSSA-AlCl₃ 70:30 21,855 25,102 PSSA-AlCl₃ 60:40 29,139 29,740PSSA-AlCl₃ 50:50 36,424 32,864 PSSA-AlCl₃ 40:60 43,709 36,446 PSSA-AlCl₃30:70 50,994 39,963 PSSA-AlCl₃ 20:80 58,279 42,198

CONCLUSION

PSSA-AlCl₃ catalysts were successfully prepared by ion exchange inliquid medium. PSSA-AlCl₃ 20:80 exhibited the best catalytic propertiesin the conversion of glucose to HMF.

Example II—Synthesis of 5-hydroxymethylfurfural from Glucose UsingPSSA-AlCl₃

The effectiveness of PSSA in the dehydration of fructose to HMF wasevaluated. PSSA was compared with a pure homogeneous catalyst (H₂SO₄)and a heterogeneous catalyst (Amberlyst 15® sulfonic resin). To bettercompare the results, the same amount of sulfonic groups (—SO₃H) was usedin each reaction. As can be seen in FIGS. 9A-9B, PSSA (which onlypossesses Brønsted acid sites) was comparable to H₂SO₄ in conversion ofglucose to HMF.

Eight catalysts with different Brønsted:Lewis acid site ratios wereprepared by ion exchange of PSSA with AlCl₃ in alcoholic medium asdescribed in Example I. 10, 20, 30, 40, 50, 60, 70, and 80% of thesulfonic groups in PSSA were respectively substituted by Lewis acidsites, able to carry out the isomerization of glucose to fructose.Samples were labelled as PSSA B:L, where B stands for the estimatedpercentage of Brønsted acid sites in the catalyst and L stands for thenumber of Brønsted acid sites estimatedly substituted by Lewis acidsites by ion exchange. To better compare the results, the same amount ofsulfonic groups (—SO₃H) was used in each reaction. As seen in FIGS.10A-10B, the addition of AlCl₃ contributes to an increased production ofHMF. The best result was obtained with the PSSA 20:80 catalyst, which isthe soluble catalyst with the highest concentration of Lewis acid sites.As seen in FIG. 10C, the PSSA-AlCl₃ 70:30 catalyst was reutilized forfive runs without a significant decline in reaction rate. Even when thevalue obtained for the 4th run was smaller than the rest of theexperiments, the activity on the 5th run was close to the mean value.The red line on the graph in FIG. 10C represents the mean value±standarddeviation for the five runs.

This Example confirms the ability of PSSA-AlCl₃ to catalyze thesynthesis of HMF from fructose and glucose in a one-pot synthesis. Lewisacid functionality was added to soluble PSSA while maintaining itssolubility for use as a homogeneous and reusable superacid catalyst inthe production of HMF from glucose. These catalysts can join theadvantages of homogeneous and heterogeneous catalysts.

Certain embodiments of the compositions and methods disclosed herein aredefined in the above examples. It should be understood that theseexamples, while indicating particular embodiments of the invention, aregiven by way of illustration only. From the above discussion and theseexamples, one skilled in the art can ascertain the essentialcharacteristics of this disclosure, and without departing from thespirit and scope thereof, can make various changes and modifications toadapt the compositions and methods described herein to various usagesand conditions. Various changes may be made and equivalents may besubstituted for elements thereof without departing from the essentialscope of the disclosure. In addition, many modifications may be made toadapt a particular situation or material to the teachings of thedisclosure without departing from the essential scope thereof.

What is claimed is:
 1. A composition comprising a compound of Formula A:

wherein m is an integer ranging from about 5,000 to about 85,000.
 2. Thecomposition of claim 1, wherein the polystyrene chain of the compound ofFormula A has a molecular weight from about 10,000 Da to about 1,500,000Da.
 3. The composition of claim 1, wherein the compound of Formula A isdissolved in a polar solvent selected from the group consisting ofwater, gamma-valerolactone (GVL), dimethyl sulfoxide (DMSO), andDMSO-water systems.
 4. The composition of claim 1, wherein the compoundof Formula A is dissolved in a polar solvent comprising water, and amethyl-isobutyl ketone MIBK+2-butanol mixture is used as an organicphase for 5-hydroxymethylfurfural (HMF) extraction.
 5. The compositionof claim 1, wherein the compound of Formula A includes Brønsted acidsites and Lewis acid sites at an estimated Brønsted:Lewis ratio of up toabout 90:10.
 6. The composition of claim 1, further comprisingnanoparticles, nanofibers, or nanosheets.
 7. The composition of claim 6,wherein the nanoparticles comprise alumina or carbon.
 8. The compositionof claim 6, wherein the nanofibers comprise carbon.
 9. The compositionof claim 6, wherein the nanosheets comprise graphene.
 10. Thecomposition of claim 1, further comprising a monomer which increases thehydrophilicity of the composition.
 11. A composition comprising apoly(styrenesulfonic acid)-based (PSSA) polymer compound having bothLewis acid sites and Brønsted acid sites, wherein the compound issoluble in polar solvents.
 12. The composition of claim 11, wherein thecompound is made by ion exchange between PSSA and one or more of AlCl₃,SnCl₄, TiCl₄, BF₃, MoS₂, ZnCl₂, VCl₄, NiCl₂, GaCl₃, GeCl₄, AsCl₂, BCl₃,SiCl₄, SbCl₃, PCl₃, or Et₂AlCl₃.
 13. A method of producing a catalyst,the method comprising adding a Lewis acid to a solublepoly(styrenesulfonic acid)-based (PSSA) polymer in a liquid medium toproduce a superacid catalyst, wherein the Lewis acid is one of AlCl₃,SnCl₄, TiCl₄, BF₃, MoS₂, ZnCl₂, VCl₄, NiCl₂, GaCl₃, GeCl₄, AsCl₂, BCl₃,SiCl₄, SbCl₃, PCl₃, or Et₂AlCl₃.
 14. The method of claim 13, wherein theliquid medium comprises a mixture of methanol and ethanol.
 15. A methodof preparing 5-hydroxymethylfurfural (HMF), the method comprising:isomerizing glucose and dehydrating fructose with a single catalyst toproduce HMF, wherein the catalyst comprises the poly(styrenesulfonicacid)-based (PSSA) polymer compound having both Lewis acid sites andBrønsted acid sites of claim 11; and, wherein the catalyst is made byion exchange between PSSA and one or more of AlCl₃, SnCl₄, TiCl₄, BF₃,MoS₂, ZnCl₂, VCl₄, NiCl₂, GaCl₃, GeCl₄, AsCl₂, BCl₃, SiCl₄, SbCl₃, PCl₃,or Et₂AlCl₃.
 16. The method of claim 15, wherein the catalyst comprisesa compound of Formula A:

wherein m is an integer ranging from about 5,000 to about 85,000. 17.The method of claim 15, wherein the isomerization and dehydration areconducted in a solvent comprising water, gamma-valerolactone (GVL),dimethyl sulfoxide (DMSO), a water-DMSO system, or a biphasicaqueous-organic system comprising water-(MIBK+2-butanol).
 18. The methodof claim 15, further comprising converting the HMF into one ofdimethylfuran (DMF), adipic acid, 1,6-hexanediol, levulinic acid,caprolactam, 2,5-dimethylfuran, 5-hydroxymethylfuronic acid,3,5-dihydroxymethylfuran, 5-hydroxy-4-keto-2-pentenoic acid, or2,5-furandicarboxylic acid (FDCA).